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Low Budget Electric Eye
by Allen Gilliam

All of us at some point in time have needed an automated way to activate a sound or light or prop, just as the patron passes a particular spot in the attraction. Or to let security know when someone is in an area that they should not be. A motion detector is the simplest way to do this, but it is not very precise. A pressure mat is a little more accurate, as long as moisture is not a concern and you have a way to cover it. The perfect solution, if you can afford it, is some type of light beam with a sensor or "Electric Eye." Off the shelf "Electric Eye" systems are expensive, so why not build your own?

A simple light beam sensor can be assembled, using inexpensive lenses and some simple electronics for less than $13. Unlike a visible laser type system, this homemade light beam will be less noticeable when it strikes your patrons' legs, and is much easier to adjust than an infrared beam that you cannot see. The sensor can be used under any ambient light level, and can be effective over as much as 25 feet.

The light beam sensor has two parts: a transmitter and a detector. The transmitter is simply a light behind a 1 inch diameter plastic lens from a toy magnifying glass. (figure 1) Make a tube the same diameter as your lens, and about 12 inches long, out of rolled up poster board or stiff paper. Cut one end of the tube at a 45% angle to form a hood, which will help conceal the light from view. After painting the tube flat black inside and out, the lens will then be glued into the tube about 1" back form the hood opening. Now build a second tube about 6 inches long, which will slide snugly inside the first tube. Seal both ends of this second tube with two circles of corrugated cardboard glued in place. Mount your light source, a pen light bulb (Radio Shack #272-1141 $1.19) or "super bright" LED (Radio Shack #276-087A $2.50), in the center of one end of the slider tube and run the wires out of the other end and to a power source.

To focus the transmitter, place a white screen of some sort where the detector will be and move the slider in and out until the brightest spot possible is on the screen. Depending on your lens and the distance of travel, the spot may be large, which is fine. The point is to get as much light as possible on to the detector. Experiment with your lens to be sure that the tube is long enough to allow the beam to be optimally focused.

The detector is constructed in the same way that the transmitter was, with a 3 inch magnifying glass lens, and a CdS (cadmium sulfide) cell (Radio Shack #276-1657 $2.29 for 5 pack) in place of the light source. (figure 2) The tube extends an inch or so beyond the lens and the inside is painted flat black to reduce reflected light coming from the side. The lens in the detector focuses the light beam from the transmitter into a single bright spot on the surface of the CdS cell. It is important to be able to both aim and focus the detector, so that the bright spot of light from the lens can be made to fall exactly onto the CdS cell. Cut a slot in the tube to look through, while you aim and focus the beam, and then cover the hole with duct tape after the adjustment. If a colored LED is used as the light source in the transmitter, a piece of glass or plastic of the same color can be placed over the CdS cell to filter any extraneous light of the wrong color.

The mounting of the detector needs to be fairly rigid. Any movement or vibration can move the bright spot off the CdS cell, rendering it inoperable. Figure 3 shows a simple yet versatile mount, which can be made from plywood scraps, and attached securely to a wall, beam, or stake driven into the ground The screws loosely slide through the first piece of wood so they can tighten against the second. Drilling a pilot hole in the second piece of wood is a good idea.

Figure 4 shows the detector circuit to detect the light beam. Q1 is a common NPN bi-polar junction transistor such as Radio Shack's "switching transistors" (#276-1617 $2.29 for 15). Potentiometer R1 turns the transistor on just enough to activate the relay (Radio Shack #275-249A $4). The voltage passed by the CdS cell when it is illuminated (when light falls on a CdS cell, its resistance drops) turns the transistor back off just enough to deactivate the relay. When the beam is broken, the relay turns on, activating your effect and the LED indicator light (Radio Shack sells a 12v LED with R4 built in, #276-209 $1.). R1 will need to be adjusted to match the lighting conditions around the detector. With the transmitter on, adjust R1 until the LED goes off. To test the circuit, wave your hand in front of the detector. As it breaks the beam, the LED should go on and then off again as your hand moves out of the light.

For best results place the transmitter and detector 6 to 12 inches above the floor or ground. It is also a good idea to hide it from view. The transmitter and detector, can be placed on opposite sides of your patron's path, or they can both be on the same side by using a mirror to reflect the beam back across. If the detector misses brief beam breaks, then it is probably on the edge of its range. Larger lenses and/or a brighter light will increase the range, and remedy this problem.

Whether you use it for prop activation, or security, this simple light beam sensor can save you a great deal of money over purchasing an off the self system electric eye. Providing you with pinpoint control over the sounds, lights and scares of your attraction.

Allen Gilliam is a self-taught home haunter from Orlando, Florida. He can be reached via email at: infidel@vol.com His website is at: http://vol.com/~infidel/halloween/ Thanks to Tony Mayse for his assistance building the original device, and his suggestions for this article

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